They could have designed a full on mask but you run into the issue of not caring who is underneath. Even the comics had Bane out of mask while in the secret 6(damn you new 52!).

Now that doesn't mean a full on mask is out. It just worked for Nolans interp.

But Bane was his own man in the movie. He knew how this all was going to go down. Understand that he,Talia, and the men all knew they were going to die when the bomb went off. Him staying to watch Batman aint exactly demonstrating he was a subordinate. Talia was fully capable of making sure a nuke went off.